1. Field of the Invention
The invention relates to the field of electrically controllable displays, and more particularly to the field of displays which change or switch color such as, for example, electrochromic display devices.
2. Description of the Prior Art
There are many uses for electrically controllable display devices. A number of such devices have been in commercial use for some time. These display devices include liquid crystal displays, light emitting diode displays, plasma displays and so on. Light emitting diode and plasma display panels both suffer from the fact that they are active, light emissive devices which require substantial power for their operation. In addition, it is difficult to fabricate light emitting diode displays in a manner which renders them easily distinguishable under bright ambient illumination. Liquid crystal displays suffer from the disadvantage that they are operative only over a limited temperature range and have substantially no memory within the liquid crystal material. Further, the visibility of many liquid crystal displays decreases as the viewer moves a few degrees off axis.
Electrochromic displays have been developed which display information through a change in the color of portions in the display via electrochemical reaction of an active material to achieve a color change. Generally, with a metal oxide as the active material, this color change is from white to a color such as blue. Because of their coloring mechanism, such displays usually require substantial power and time to write or erase displayed information. The quantities of power required are undesirably large, especially for battery operation. Moreover, the time required to change displayed information makes such materials unacceptable for many display applications. None of these displays shows more than a single color against a background. This limits the versatility of such displays since the variation of color of a character cannot be used to convey additional information.
Rare-earth diphthalocyanines are known from prior publications to have electrochromic properties in which the color of the diphthalocyanine can change over a period of about eight seconds upon application of a potential difference across an electrochemical cell having a diphthalocyanine film on one of the electrodes. P. N. Moskalev and I. S. Kirin, "Effect of the Electrode Potential on the Absorption Spectrum of a Rare-Earth Diphthalocyanine Layer," Opt. i Spektrosk, 29, 414 (1970) and P. N. Moskalev and I. S. Kirin, "The Electrochromism of Lanthanide Diphthalocyanines," Russian J. Phys. Chem., 46, 1019 (1972).
U.S. Pat. No. 4,184,751 of M. M. Nicholson, the inventor herein, describes the use of metal diphthalocyanine complexes as the electrochromically active material in an electrochromic display cell. Rapid color changes in less than 50 milliseconds are achieved, thus alleviating the slow switching time previously reported for rare-earth diphthalocyanine complexes. Power requirements are small because of the low power switching characteristics of the display material and because the display exhibits an open circuit memory of from several minutes to several hours, depending on its construction. A multi-color, i.e., more than one color, display is achieved through use of a range of voltages applied between display and counter electrodes. Color reversal of displayed information and the background against which it is displayed is achieved through use of display electrodes in the background portions of the viewing area as well as in the character segments.
Matrix display devices contain one or more arrays of many small elements or dots of color-changing material that can be selectively activated or switched to form virtually any alphanumeric or graphic pattern. To create such patterns and erase them at will, a means must be provided to address each element independently without activating those in the surrounding area. An integrated drive matrix of thin-film transistors could be built into the display device for this purpose so that each element is provided, in effect, with a separate switch connecting it to the power supply. See T. P. Brody and P. R. Malmberg, "Large Scale Integration of Displays Through Thin-Film Transistor Technology," Int. J. Hybrid Microelec., II, 29 (1979).
Although the use of an integrated drive matrix is an elegant general-purpose approach for matrix displays, its fabrication is relatively complicated. When possible, electronic engineers prefer to use the simpler multiplexed addressing scheme of the sort which uses two sets of parallel conductive, linearly-extending electrodes disposed at right angles. A thin layer of the display material and any associated component such as an electrolyte is disposed between the two sets of electrodes. When a display system is fully amenable to such addressing, a single selected dot or element can be activated by a signal applied across one electrode in each of the two sets of orthogonal electrodes.
But the use of multiplexed addressing with a matrix display using electrochromic materials can cause the loss of some of the inherent advantages of these materials. Even though electrochromic displays exhibit appropriate voltage threshold characteristics for good resolution of the individual elements or dots, the advantages of a potentially outstanding feature of such displays, memory or pattern retention, may be lost if the displays are multiplexed as described above. This is particularly true of a multicolor electrochromic system that provides different colors by undergoing both oxidation and reduction reactions. Memory or pattern retention is lost because of galvanic action between oppositely charged elements in contact with the same electronic conductor and electrolyte. The elements discharge to intermediate electrical and color states thereby erasing the information. A similar problem exists in a multiplexed electrochromic display containing a combination of a redox couple and a soluble switching material. See Arellano et al., "Matrix Addressed Electrochromic Display," U.S. Pat. No. 4,146,876 (1979). The loss of memory problem is overcome in the Arrelano et al. approach by frequent refreshing of the color through the use of a repetitive input signal. Unfortunately, this increases the power consumption.